This invention relates to ink jet printers. More particularly it relates to ink jet printers of the type which form drops from a stream or streams of ink, charge the drops and then deflect them onto a substrate to be marked. Such devices are well known in this art.
Printing distortions have long been a problem with ink Jet printers. Many of the causes are well known and include variation in ink quality, improper jet nozzle drive and improper proximity to the substrate. Recent research has indicated another important cause of distortion exists, which emanates from non-proportional changes in charging.
Ink jet printing devices typically employ a programmable controller (PC) to set the various parameters necessary for proper operation. The PC includes memory containing drop position compensation data for each graphic or alpha-numeric character to be printed. This data is created at the factory when the jet stream is carefully centered within the charging electrode. This data is used on all ink jet machines of the same model. Actual printers, for one reason or another, tend to have their ink streams misaligned within the electrode or have their drop spacing or electrode width somewhat out of specification. Indeed, mechanical stream alignment is difficult to accurately achieve in the field. If the stream is not well aligned distortions in ink jet printing will occur such as illustrated in FIG. 4. This problem is particularly evident when every drop printing is employed, and to a lesser extent with every other drop printing. The present invention teaches various ways by which these distortions can be corrected. In addition, the invention can compensate for changes in electrical resistivity in the ink. This invention is particularly important for use in multi-jet printers to maintain consistent quality throughout an array of jets.
It is accordingly an object of the invention to provide a method and apparatus for correcting printing distortions which occur in ink jet printers due to non-proportional changes in charging of the drops.
Another object of the invention is to provide improved ink jet printing of characters on a substrate by adjusting the charge voltages to accommodate machine to machine variations.
A further object of the invention is to provide an automatic compensation method for field calibrating ink jet printers to maintain high print quality. These and other objects of the invention will be apparent from the remaining portion of the specification.